The present invention relates generally to magnetic resonance (MR) imaging systems. More particularly, the present invention relates to an MR imaging system equipped to reduce image artifacts caused by magnet vibrations produced therein.
When an object of interest, such as human tissue, is subjected to a uniform magnetic field (polarizing field B.sub.0, along the z direction in a Cartesian coordinate system denoted as x, y, and z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it at their characteristic Larmor frequency. If the object, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated or "tipped" at a certain tipping angle, into the x-y plane to produce a net traverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation field B.sub.1 is terminated, and this signal may be received and processed to form an MR image.
When utilizing these signals to produce MR images, linear magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) are also employed. Typically, the object to be imaged is scanned by a sequence of measurement cycles in which these gradient waveforms vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals, also referred to as MR signals, are digitized and processed to reconstruct the image using one of many well-known reconstruction algorithms.
Ideally, a uniform magnetic field (B.sub.0) and perfectly linear magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) would be utilized to image the object of interest. In reality, however, perturbation to the magnetic field, such as eddy currents, gradient amplifier infidelity, gradient non-linearity, magnetic field inhomogeneity, and Maxwell terms, can exist, resulting in image artifacts such as blurring, distortion, ghosting, and shift in the reconstructed MR image. In recent years, as magnets included in MR imaging systems have become smaller in size and weight in order to reduce cost, another perturbation factor is emerging as an important source of image artifacts.
As magnet size and weight are reduced, magnet vibration is becoming an increasingly serious problem. Magnet vibration causes perturbation magnetic fields, i.e., magnetic fields with vibration components, to be applied to the object of interest. In turn, these vibration components produce undesirable image artifacts in the reconstructed MR image. Constrained by cost, it is often difficult to proactively design magnets to completely eliminate all critical vibration components.
Thus, there is a need for an MR imaging system capable of correcting or compensating for image artifacts caused by magnet vibration before reconstructing an MR image. In order to do so, there is a need for an MR imaging system capable of quantifying the magnetic field vibration components.